The present invention relates to a method and apparatus for severing packaging material, and more particularly, for severing packaging material which extends between successive wrapped loads during a continuous wrapping process.
Stretch wrapping is a method of wrapping loads with a tensioned web of packaging material such as stretch wrap film. An example of a method and apparatus for stretch wrapping is disclosed in U.S. Pat. No. 4,317,322 to Lancaster et al., assigned to Lantech, Inc., and is incorporated herein by reference.
Recently, developments in stretch wrapping have focused on methods and apparatus for continuously wrapping a series of loads in an uninterrupted manner. Such continuous wrapping involves conveying a series of loads through a wrapping area in which the loads are spirally wrapped with stretch wrap film. As a result, the series of wrapped loads become encased with a continuous tube of stretch wrap film. A severing bar, positioned downstream of the wrapping station, is passed between successive wrapped loads. The severing bar severs the tube of packaging material between successive wrapped loads and separates the wrapped loads.
As it has become possible to wrap at higher speeds, the conveying speed of the wrapped loads has also increased. To prevent the servering bar from striking the rapidly moving loads, it has become necessary to translate the severing bar in the conveying direction along with the loads while passing it between successive wrapped loads. In contrast to merely translating the severing bar back and forth along a single linear path, the requirement of translating the severing bar in the conveying direction with the loads during severing has required a system which transports the severing bar around a closed path having at least two different segments. Both segments have a component in the conveying direction. In the first segment, the severing bar passes across the load path between and along with a first pair of successive loads. In the second segment the severing bar returns across the load path between and along with a second pair of successive loads.
In order to reliably sever the packaging material on a prolonged and repeated basis, a heating element is incorporated in the severing bar to heat the severing bar so that the packaging material is melted or substantially weakened upon coming in contact with the severing bar. The heating element in the severing bar is supplied with electricity by an electrical conductor which is connected to a power source.
While the earlier linearly reciprocating severing bar did not twist the electrical conductor during operation, the more advanced systems, which transported the severing bar in the conveying direction along a closed path during the severing operation, would twist the electrical conductor in an unsatisfactory manner during repeated cycling of the severing bar along the closed path.
As a result, various commutator systems, such as those shown in FIGS. 2 and 3, were developed in order to conduct electricity from a fixed central point to the heating element in the severing bar as it proceeded around the closed path. The commutator systems prevented twisting of the electrical conductor during repeated cycling of the severing bar along the closed path.
However, during operation of the severing bar, the commutator systems are exposed to considerable forces, especially when the severing bar is changing directions. As a result of these forces and the high number of repetitions required in fast continuous packaging systems, the commutators failed and required replacement after a relatively short period of time.
Such need for frequent replacement of the commutators has greatly increased the expense of operating the continuous packaging systems. In addition to the cost of the parts and services, there can be significant losses in time, money and effectiveness during the periods of time during which the system is required to be idle due to such failures. As a result, the advantages obtained through the development of such sophisticated high speed wrapping machines have been substantially subverted by decreases in efficiency due to extended and repeated periods of downtime due to failures of the commutator systems.
Accordingly, it is an object of the present invention to provide a method and apparatus for severing packaging material in which electricity can be supplied to a heating element in a severing bar in a reliable and failureproof manner to prevent downtime and the need to service and replace parts.
It is a further object of the present invention to provide a method and apparatus for severing packaging material which in which the conductors for heating the severing bar are not twisted during repeated cycling of the severing bar around a closed path.
It is an additional object of the present invention to provide a method and apparatus for severing packaging material in a system which is simple to construct and easy to operate.
It is another object of the present invention to provide a method and apparatus for severing packaging material during a continuous wrapping process which does not require the use of commutators for preventing twisting of the conductor for the heating element.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.